Combi boilers have two functions: central heating and domestic hot water. Sometimes one fails while the other still works. If your heating is fine but hot water has failed, here's how to diagnose it.
Check 1: The diverter valve
Combi boilers use a diverter valve to switch between heating and hot water mode. A stuck or faulty diverter valve is one of the most common causes of hot water failure on an otherwise functioning combi. Signs: heating works but hot water is cold, or there's a delay before hot water arrives.
Check 2: Heat exchanger blockage
In hard water areas (like Northamptonshire), limescale can partially block the secondary heat exchanger (domestic hot water side). Flow rate gradually reduces, then stops. Descaling can restore function; severe scaling may require heat exchanger replacement.
Check 3: The plate heat exchanger
Most modern combis use a plate heat exchanger for domestic hot water. These can scale, crack, or fail. A failed PHE means cold water, but the heating circuit still works.
Check 4: Hot water temperature sensor
If the sensor reads an incorrect temperature, the boiler may decide hot water is already hot enough and not fire. Engineer diagnosis required.
What to do
Most of these require a Gas Safe engineer to diagnose and repair. We diagnose and carry diverter valves, plate heat exchangers, and sensors for all major brands. Book a repair visit.